Heating pads are normally used to apply heat to parts of the body to aid in healing or for comfort. Most consist of a remote control device and a heating device. The remote control is typically plugged into a household AC power outlet and regulates the temperature of the heating device. The remote control has a user interface, typically pushbuttons or a multi-position switch, so the user can turn the heating device power on or off and select a desired temperature. The heating device consists of a heating means, usually a flexible insulated wire, a means of preventing unsafe temperatures, and a cover to prevent user contact with the electrical components.
One method of controlling temperature is to use a thermostat that opens the heating power circuit when a fixed temperature is reached. However, such thermostats can be relatively expensive, costing approximately twenty-two cents each. In addition to heat from the heating wire, an additional source of heat is applied to the thermostat by means of a resistive heating source with a power set by the remote control. A low heat is achieved by passing all of the current that is flowing through the heating wire through the additional heating source, causing the thermostat to open at a lower temperature than it would if it were heated by the heating wire alone. To achieve the highest desired temperature, no power is applied to the resistive source, and for a mid range temperature power is applied to the resistive heating source for one-half cycle of the AC line.
Typically a second or third thermostat is used in separate sections of the heating pad to prevent unsafe temperatures from occurring due to uneven heating. Such an unsafe condition can occur when a portion of the pad is covered, preventing heat from radiating. The additional thermostats open the heating wire circuit before an unsafe temperature occurs. In these cases, the temperature is monitored in only limited locations and the measured temperature is not representative of the average temperature of the heating pad. Further, thermostats generate electrical noise when they open and close and are expensive.
Another method of controlling temperature is to use a separate sensing wire that is concentrically wound around the heating wire. The resistance of the sensing wire varies with temperature by a known amount. Therefore, by sensing the wire resistance, the temperature can be calculated. The control device constantly monitors the sensing wire resistance and calculates the wire temperature. If the temperature is above the user-selected value, power is removed from the heating wire. If the temperature is below the user-selected value, power is applied to the heating wire.
Because the sensing wire is distributed over the entire heating wire, its resistance is a measure of the average temperature of the wire. To prevent localized hot spots from reaching a dangerous temperature, the insulation between the heating and sensing wires is made of a material that melts or becomes electrically conductive when unsafe temperatures are approached. A circuit in the remote control monitors the resistance between the heating and sensing wires and removes power from the heating wire if the resistance falls below a value corresponding to an unsafe temperature.
The sensing wire resistance changes are small, typically 0.25% per degree Fahrenheit. The variation in the room temperature resistance of the sensing wire is large enough that an initial calibration must be done on each heating pad, thereby adding manufacturing expense. Further, the pad must be discarded and replaced if localized heating occurs that melts the insulation between the heating and sensing wires. Moreover, the sensing wire does not accurately sense the pad temperature because it is in close proximity to the heating wire, and is separated from the surface of the pad by the outer insulation layer and the pad cover components.